hi, my LG plasma was working fine until the other day, when it just went blank. Then on turning it on again (via the remote), it just displays the LG welcome screen, and pressing any buttons on the TV or on the remote, fails to do anything.

I disconnected the power plug overnight and it worked the next day, but now it's done it again ... stuck on the LG welcome screen.

Unplugging power is only reset you can do. Main board is borked, will have to be replaced. May be different versions so check part number on your board before ordering. Check eBay or google part number. Here is one EXAMPLE Main Board.